Mind-Controlling Parasites and The Last of Us

Though the exact process O. unilateralis uses remains a mystery, the purpose of it is clear: placing the ants this way makes reproduction more efficient. With the fruiting fungus suspended in air, its spores can travel further, giving them a greater chance to infect another host. It's an effective tactic, too. O. unilateralis can wipe out entire ant colonies. The infection is so dangerous that when worker ants recognize signs of infection, they carry the infected ant as far away from the colony as possible and abandon their convulsing bodies to prevent the fungus spreading. In The Last of Us, despite all humanity's advancements they deal with their own Cordyceps infection not much differently from the ants - take the infected, shoot them, dump them and forget them.

What's really interesting, however, is how much attention to detail Naughty Dog paid to fleshing out the life cycle of the infected - and their ability to control their hosts. Though the infection is principally based on O. unilateralis, in reality it's more of an amalgam of different parasites that practice behavior manipulation. After all, many organisms ranging from viruses to fungi propagate themselves by playing puppeteer with their unwilling partners. The phenomenon is common enough to have its own fledgling branch of science known as neuroparasitology, and scientists are just beginning to understand the mechanical and chemical processes parasites use to control their hosts. Some parasites cause the host to be attracted to light, others produce dopamine or increase hormone production, while some alter the central nervous system - others, like O. unilateralis, are still a mystery.

In any case, most organisms use behavior manipulation as a tool in their reproductive cycle. The jewel wasp, for example, will attack a cockroach and inject its venom into the unlucky bug's brain, blocking its escape reflex. The wasp then takes the cockroach's antennae like a leash and leads the docile creature to its burrow to become a living vessel for its eggs. Hairworms, which live in water, actually gestate on land inside cockroaches, katydids and beetles. When the worm attains its full growth, it mimics the host's neuromodulators, driving the insect to commit suicide in water where the parasite can wriggle free and begin adulthood. Others trick mosquitos into drinking less blood from each source, causing them to seek out more sources of blood and therefore increasing the chance of transmission. Vertebrates aren't immune to the neuroparasite's spell, either. The feline parasite Toxoplasma gondii actually gets to its preferred host through infecting rats and mice, then altering the host's brain so that the rodent becomes attracted to, rather than afraid of, the smell of cat urine. When the cat consumes the confused rodent, it ingests the parasite as well.